1. Field of the Invention
The principles of the present invention generally relate structures of backlight units in a direct type liquid crystal display device.
2. Discussion of the Related Art
As the capabilities of information processing apparatuses and display technologies continue to advance, various types of display devices are actively developed. Due to their low power consumption and compact, lightweight construction, liquid crystal display (LCD) devices have been the subject of extensive research and have been implemented in many applications.
Generally, LCD devices include LCD panels that include a thin film transistor (TFT) array substrate, a color filter substrate spaced apart from the TFT substrate, and liquid crystal material provided within a gap formed between the TFT array and color filter substrates. The TFT array substrate generally supports a plurality of gate lines and a plurality of data lines crossing the plurality of gate lines, wherein unit pixels are defined at the crossings of the gate and data lines, and switching devices (i.e., TFTs) connected to the gate and data lines at the crossings thereof. The color filter substrate generally supports color filter layers (e.g., red, green and blue layers) to express images having natural colors.
By themselves, LCD panels do not generate light that is necessary to express images. Therefore, to express images, light must be generated by a light source that is external to the LCD panel. In many cases, such a light source is provided within a backlight unit. Depending on the location of the light source with respect to the LCD panel, backlight units can be generally classified as either direct-type or edge-type.
Edge-type backlight units generally include a lamp unit provided along a lateral side of a light-guide plate that is disposed beneath an LCD panel and are typically incorporated within relatively small-sized LCD devices (e.g., monitors for laptop, desktop computers, etc.). Direct-type backlight units generally include a plurality of lamps provided beneath a lower surface of a light-diffusion sheet that is, in turn, disposed beneath an LCD panel and are typically incorporated within large-sized LCD devices (e.g., large monitors, televisions, etc.).
FIG. 1 illustrates a cross-sectional view of a related art direct-type backlight unit.
Referring to FIG. 1, disposed at a backside of an LCD panel 120 a related art direct-type backlight unit generally includes a plurality of lamps 110 (i.e., bar-type, cold cathode fluorescent lamps (CCFLs)) arranged in parallel with one another and disposed within a cavity formed in a bottom cover 101 (i.e., a mold frame) for protecting the plurality of lamps 110. A reflective plate is disposed between the bottom cover 101 and the plurality of the lamps 110 to reflect light generated by the lamps 110 toward the LCD panel 120. The related art direct-type backlight unit further includes a diffusion plate 107 for uniformly diffusing the light generated by the lamps 110 and transmitting the diffused light toward the LCD panel 120, and a diffusion sheet 109 disposed on the diffusion plate 107. The diffusion plate 107 and diffusion sheet 109 are supported by edges of the bottom cover 101.
FIG. 2 illustrates a perspective view of the related art direct-type backlight unit shown in FIG. 1.
Referring to FIG. 2, the plurality of lamps 110 are supported within the bottom cover 101 by lamp fixing members 111. Moreover, lamp electrodes (not shown) formed on opposing ends of the plurality of lamps 110, are supported by a lamp support 112 provided at opposing sides of the bottom cover 101. Each lamp electrode is connected to power supply lines (not shown) for supplying power to corresponding ones of the plurality of lamps 110. Although not shown, power supply lines that supply power to a particular lamp are connected to each other to form a closed circuit around the particular lamp 110. Each closed circuit is provided with an inverter to convert externally applied direct current (DC) voltage into an alternating current (AC) voltage that is supplied the particular lamp 110. Although not shown, each inverter is connected to a ground printed circuit board (PCB) arranged at a rear. surface of the bottom cover 101.
As noted, the plurality of lamps 110 discussed above with respect to FIGS. 1 and 2 are provided as CCFLs. Each CCFL can be conceptually divided into a fluorescent part and external electrode parts for applying a tube voltage to the fluorescent part, wherein the fluorescent part generates light in response to the applied tube voltage. The fluorescent part generally includes a fluorescent material coated onto the interior of a lamp tube. External electrodes within the external electrode parts are typically formed by coating an opaque metal film onto opposing ends of the lamp tube. Generally, the brightness of a lamp 110 increases when the applied tube voltage increases. However, when the applied tube voltage is over 2000 Vrms, an undesirably excessive amount of ozone can be generated within the fluorescent part due to ionization of discharge gas within the lamp 110. Excessive amounts of ozone deleteriously reduce the brightness of the lamp 110. Therefore, the external electrodes of the lamp 110 can be lengthened to reduce the applied tube voltage.
FIG. 3 illustrates a plan view of a related art direct-type backlight unit incorporating lamps with lengthened external electrodes. FIG. 4 illustrates a side view of the related art backlight unit shown in FIG. 3.
Referring to FIGS. 3 and 4, a plurality of lamps 110, each conceptually divided into external electrode parts 103a and 103b and a fluorescent part 105, are supported by a plurality of lamp supports 112. When the lamps 110 are installed within the lamp supports 112, the lengthened external electrode parts 103a beneficially reduce the applied tube voltage but undesirably extend into a luminous area of the backlight unit between the lamp supports 112. However, because the external electrode parts 103a and 103b are typically formed by coating an opaque metal film onto the lamp tube, the lengthened external electrode parts 103a and 103b uniformly extend into the luminous area of the backlight unit and undesirably cast shadows onto portions of the diffusion plate 107 and diffusion sheet 109 that are aligned with display regions of an overlying LCD panel. The cast shadows, therefore, decrease the uniformity of light transmitted to the LCD panel as well as decrease the average brightness of images expressed by the LCD panel.